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Physics > Magnetic Fields > Flashcards

Magnetic Fields Flashcards

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AP® Physics flashcards Physics 101 flashcards
1
Q

What direction do magnetic field lines go from and to?

A

From North to South pole of a magnet.

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2
Q

What happens when current flows through a wire or any other straight long conductor?

A

A magnetic field is induced around the wire.

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3
Q

How do you work out the direction of a magnetic field around a current-carrying wire?

A

Use the right-hand rule.

hand in fist with thumb pointing upwards, thumb is direction of current and fingers are the direction of field lines

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4
Q

What is a solenoid?

A

If you loop a current-carrying wire into a coil in one plane, the surrounding magnetic field is doughnut shaped, while a coil with length (a solenoid) forms a field like a bar magnet.

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5
Q

What happens if you put a current-carrying wire into an external magnetic field?

A

The field around the wire and the external field are added together, causing a resultant field which cause a ‘pushing’ force on the wire.

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6
Q

How do you work out the direction of the current, the direction of the external magnetic field or the direction of the force on the wire?

A

Flemming’s Left-Hand Rule.

Thumb = motion (force), first finger = magnetic field (uniform), middle finger = current

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7
Q

What is the definition of magnetic flux density (B)? What are the units of magnetic flux density?

A

The force on one metre of wire carrying a current of one amp at right angles to the magnetic field. Units are Tesla (one newton per amp per metre).

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8
Q

What is the equation for force on a current-carrying wire at 90 degrees to a magnetic field and what does each letter mean? Is this force a maximum or a minimum and why?

A 
F = BIl
F - Force / N
B - Magnetic flux density / T
I - Current through wire / A
l - Length of wire / m

This force is a maximum as the wire could not be at right angles, in which case the force would be smaller.

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9
Q

What experiment could you do to investigate the force on a wire in a magnetic field?

A
  • Pass wire connected to support on a set of scales through two parallel magnets (uniform magnetic field)
  • When you pass a current through wire, there will be a force downwards giving a reading on the scales
  • Repeat this for a range of current
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10
Q

What would you do with the results of the wire with magnets experiment to investigate force on a wire in a magnetic field?

A

-You would convert your mass readings into force by using F = mg, and plot your data on a graph of force against current

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11
Q

What does the graph of force against current look like? What is the gradient equal to and what can you use this for?

A

Straight line through the origin.
Gradient equals Bl, therefore you can divide the gradient by the constant length of the wire to get the magnetic flux density, B.

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12
Q

What other factors could you investigate to do with force on a wire in a magnetic field? (other than the current in the wire)

A
  • Length of the wire

- Flux density

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13
Q

How do you derive the F = BQv equation?

A

Use the F = BIl equation:
I = Q/t
l = vt
insert these values

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14
Q

How does Fleming’s Left Hand Rule work for charged particles?

A 
Second finger (normally current) is motion of positively charged particle.
If it's negatively charged, point finger in opposite direction of motion.
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15
Q

How do you draw a magnetic field going into/out of the page?

A

Going into - circle with a cross in

Out of - circle with a dot in the middle

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16
Q

How can you find the radius of the circular path followed by charged particles in a magnetic field?

A

Combine the equation F = BQv and F = mv^2 / r

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17
Q

What is the equation for frequency of rotation of an object in circular motion?

A

f = v / 2πr

because 2πr is the distance it travels in each rotation (circumference)

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18
Q

How can you get an equation for the frequency of rotation in terms of B, Q and m?

A

Combine the equation f = v / 2πr and r = mv / BQ

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19
Q

What is a Cyclotron and what is it made up of?

A

A Cyclotron is a type of particle accelerator and it is made up of two hollow semi circular electrodes with a uniform magnetic field applied perpendicular to the plane of the electrodes, and an alternating potential difference applied between the electrodes.

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20
Q

What happens when a charged particle is fired into an electrode in a Cyclotron?

A

The magnetic field makes them follow a (semi) circular path and then leave the electrode. An applied potential difference between the electrodes then accelerates the particle across the gap until it reaches the other electrode.

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21
Q

What happens to the radius of the circular path when a particle in a Cyclotron is accelerated?

A

It follows a larger radius due to an increase in velocity.

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22
Q

Why is the potential difference switched each time a particle passes between electrodes in a Cyclotron?

A

So that the particle is accelerated again before entering the next electrode.

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23
Q

What happens when the particle in a Cyclotron is accelerated lots?

A

It eventually reaches a radius where it can exit the Cyclotron.

24
Q

What does the fact that the frequency of circular motion doesn’t depend on radius mean for the potential difference?

A

The alternating potential difference can have a constant frequency as the particles will spend the same amount of time in the electrodes each time they are accelerated.

25
Q

What is magnetic flux?

A

The amount of magnetic flux density passing through an area.

26
Q

What is electromagnetic induction?

A

If there is relative motion between a conducting rod and a magnetic field, electrons in the rod will experience a force, which causes them to accumulate at one end, inducing an electromotive force (e.m.f)

27
Q

What is flux linkage?

A

When a wire coil is moved in a magnetic field, the size of the e.m.f induced depends on the magnetic flux passing through the coil , and the number of turns on the coil cutting the flux (Flux linkage = number of turns on coil x magnetic flux = BAN)

28
Q

What will a change in flux linkage of one weber per second induce?

A

An electromotive force of 1 volt in a loop of wire.

29
Q

How would you work out the flux linkage if the magnetic flux is not perpendicular to the area you’re interested in (at an angle)?

A

You use trigonometry to split up the magnetic flux into components which are parallel and perpendicular to the area.

30
Q

State an experiment you could carry out to investigate flux linkage.

A

You could use a search coil with a stretched metal spring acting as a solenoid around it, which is connected to an AC power supply and an oscilloscope to view the values.

31
Q

In the search coil experiment, what does having an AC current mean?

A

The magnitude of the solenoid is constantly changing, meaning the flux through the search coil is changing which can induce an e.m.f.

32
Q

What do you do with the search coil to investigate flux linkage?

A

Position it inside the solenoid and orientate it so that it is parallel to the solenoid, and it’s area is perpendicular to the field, and record the induced e.m.f.

33
Q

What do you do after you have recorded the induced e.m.f when the search coil is parallel to the solenoid?

A

Rotate the search coil by 10 degrees and record the induced e.m.f again. Repeat this until search coil is rotated by 90 degrees.

34
Q

What do you find from the search coil experiment and what graph can you plot?

A

You find that as you rotate the search coil, the e.m.f decreases, because the search coil is cutting fewer flux lines. If you plot a graph of induced e.m.f against angle of rotation, you should find that the induced e.m.f is a maximum at zero degrees and zero at 90 degrees.

35
Q

What is Faraday’s Law?

A

Induced e.m.f is directly proportional to the rate of change of flux linkage.

36
Q

What is the gradient of a flux linkage - time graph equal to?

A

The magnitude of the e.m.f.

37
Q

What is the area under the graph of magnitude of e.m.f against time equal to?

A

The change in flux linkage.

38
Q

What is Lenz’s Law?

A

The induced e.m.f is always in such a direction as to oppose the change that caused it.

39
Q

What does the flux linkage alternate between in a rotating coil?

A

+ and - BAN, as the angle changes.

40
Q

What are the two ways of changing the graph of induced e.m.f against time?

A

Changing the speed (more speed = higher max e.m.f) or changing the size of the magnetic field (bigger flux density = bigger maximum e.m.f)

41
Q

How do generators work?

A

They induce an electric current by rotating a coil in a magnetic field.

42
Q

What is alternating current?

A

A current which changes direction with time, meaning the voltage across a resistance goes up and down in a regular pattern. (positive to negative)

43
Q

What three pieces of information can you get from an AC oscilloscope trace?

A

Time period, peak voltage and peak to peak voltage.

44
Q

How do you work out the r.m.s power for an AC supply?

A

Irms x Vrms

45
Q

What is a transformer?

A

Transformers are devices that make use of electromagnetic induction to change the size of the voltage for an alternating current.

46
Q

How does a transformer work?

A
  • AC current causes iron core to magnetise, demagnetise and re-magnetise continuously in opposite directions, causing a rapidly changing magnetic flux
  • Rapidly changing flux in iron core passes through a second coil with more or less turns than the first coil
  • This either steps the voltage up or decreases it
47
Q

What are eddy currents?

A

Metallic core in a transformer is being cut by continuously changing flux, which induces an e.m.f in the core. In a continuous core this causes currents called eddy currents which cause it to heat up and energy to be lost.

48
Q

What effect do eddy currents have in transformers?

A

They cause inefficiencies due to energy lost through heat.

49
Q

How can you reduce the effect of eddy currents?

A

Laminate the core so a current can’t flow through layers of the core.

50
Q

What are two ways, other than eddy currents, that heat energy is lost in a transformer? How can these two be reduced?

A
  • Heat generated by resistance in coils. Can be reduced by using thick copper wire as it has low resisitivity and a larger diameter meaning smaller resistance
  • Heat generated by energy needed to magnetise and demagnetise the core. Can be reduced by using a magnetically soft material that magnetises and demagnetises easily
51
Q

How can you reduce the magnetic loss from the coils being far apart in a transformer?

A

Have the coils as close as possible, which can include winding the coils on top of each other or around the same part of the core.

52
Q

How can you find the energy lost in an inefficient transformer?

A

Use the efficiency equation to work out the useful power, and then use E = Pt to work out the energy.

53
Q

Why do cables in the national grid carry a high voltage but a low current?

A

To reduce resistance and therefore reduce energy loss as much as possible.

54
Q

What voltage do step-up transformers in the national grid raise the voltage to? What does this mean for the current?

A

About 400,000V, meaning a very low current.

55
Q

How would you investigate the relationship between the number of turns and the voltages across the coils of a transformer?

A
  • Put 2 C-cores together and wrap wire round each to make the coils (5 turns in first and 10 in second)
  • Turn on AC supply and record the voltage across each coil
  • Repeat for different ratios of turns but the same voltage
56
Q

How would you investigate the relationship between current and voltage of the transformer coils for a given number of turns in the coil?

A
  • Put 2 C-cores together and wrap wire round each to make the coils and add a variable resistor to the primary coil and an ammeter to both circuits
  • Turn on power supply and record current and voltage across each coil
  • Leave number of turns constant and adjust the the variable resistor to change input current
  • Record current and voltage for each coil, then repeat with a range of input currents

Physics (8 decks)

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